This invention relates to degasification of coal seams, and more particularly to a method for degasifying a coal seam which is in proximity to an underlying or overlying coal seam.
Underground coal mining has always been subject to explosions caused by methane gas which is always present to some extent in the working area. In cases where the coal seam being mined is adjacent an overlying or underlying coal seam, methane gas from the overlying or underlying coal seam can infiltrate into the working area and aggravate the methane problem.
Several techniques have been used in the past to control the methane concentration in coal mines. The most common method is air ventilation to dilute the methane in the mine. More recently, the removal of methane through the use of boreholes drilled into or through a mineable coal seam has received considerable attention. A discussion of the prior art methods or removing methane utilizing boreholes drilled into or through coal seams is found in U.S. Pat. No. 3,934,649. As discussed therein, vertical boreholes from the surface have been drilled into coal seams, but their efficiency has been relatively low due to their failure to intersect the major natural fractures in the coal so that significant quantities of methane can be released. Also discussed therein is the technique of drilling horizontal boreholes through the coal seam. These horizontal boreholes have been quite effective in some cases, and their use is increasing.